[headlight]

If you go with an optical sensor I would consider putting them in the path of the airflow so hopefully the dust gets blown off. Remember, you will probably have dust sticking to the plastic due to static cling, so just angling it might not be enough.

I would personally recommend the weight sensor. Forget the load cell, it doesn't have to be fancy, just tip one of the feet up and put some springs under it with a mechanical limit switch, adjust the springs until the switch makes when the bin is approximately full. It won't require the use of a micro controller or other electronics, and will be easy for other people to adjust and trouble shoot if you're not there. Additionally you can probably make most of it out of wood, which I assume is what the shop does.